Bird of the Month: Mountain Blue Bird (Sialia currucoides)

What is the secret to this eye-catching bird's brilliant blue coloring?

By Andy McCormick

Mountain Bluebird Sialia
currucoides

Length 7.25” Wingspan 14”

Weight 1 oz (29 g)

AOU code: MOBL

The Mountain Bluebird, sometimes
called the “prairie bluebird” (Dunne), is a bird of open grasslands. It is seen
most frequently in Central and Eastern Washington. It enjoys a wide range of
habitats including an altitudinal range from grasslands to open areas in
forests, to alpine tundra (Alderfer).

Electric Blue – Structural
Coloration

The serene cerulean blue of the
breeding male Mountain Bluebird is what people love about this bird. It is
captivating, and on sunny days, it is brilliant. The color results from the
interaction of light with the structure of the feathers. This phenomenon is
limited to the colors blue, violet, and indigo, which have short wave-lengths.
When light is reflected from certain structures in the bird’s feathers only the
short wave-length colors are reflected and the human eye sees blue. “No blue
pigments have ever been discovered in the feathers or skin of birds” (Hill).
Bluebirds are quite beautiful and “many people view bluebirds as an emblematic
species representing all that is good in the world” (Power and Lombardo).

This color of the male is also
important in mate selection. Studies have shown that females prefer brighter
blue males. Female Mountain Bluebirds will select additional males for
extra-pair copulations. Researchers have observed that the most brilliant blue
males will sire more extra-pair offspring, inferring that females choose
brighter males to be their mates (Hill).

Cavity-nesting Thrush

The Mountain Bluebird is a
cavity nester and will build a cup nest made of twigs, weed stems, grass and
sometimes animal hair or feathers in a natural hollow, an old woodpecker hole,
in crevices in rocks, and holes in the sides of buildings. Many nest boxes have
been erected to support these and other bluebirds.

Typically, four to six pale blue
eggs are deposited. The female incubates them for about two weeks and
hatchlings are ready to leave the nest in another three weeks. The young are
cared for by the parents for another three to four weeks. Mountain Bluebirds
often have two broods per year (Kaufman).

Kestrel-like Hovering

The diet of the Mountain
Bluebird consists primarily of insects including beetles, grasshoppers,
caterpillars, crickets, ants, and bees (Kaufman). Foraging usually begins from
a perch such as a fence, low branch, or rock. Frequently the Mountain Bluebird
will hover in place as if to inspect an area before dropping to capture prey,
in a manner that is very similar to a hunting American Kestrel. At other times,
it will attempt flycatching and hawking insects in flight (Power and Lombardo).
You can see a video of a male Mountain Bluebird bringing food to a nest box at the Macaulay
Library.

Mountain Bluebirds are migratory
along the Pacific Flyway and breeding birds will arrive in Washington in late
March or early April. However, many Mountain Bluebirds breed farther north and
will be observed as migrants passing through the state. They begin southward
migration in September as they gather in family flocks and later join larger
flocks of a hundred or more on their way south. Many will associate with
Western Bluebirds and sparrows in mixed migratory flocks (Power and Lombardo).

The Mountain Bluebird is in the
genus Sialia, Greek for a bird, and shares it with the Western (S.
Mexicana) and Eastern Bluebirds (S. sialis). There are no
large-scale conservation measures in place and the population is considered
stable or expanding. Breeding has been supported by human activities such as
controlled burns and the introduction of nest boxes, which Mountain Bluebirds
use more than other bluebirds. It is a species of least concern on the
International Union for Conservation of Nature Red List.